You can't really do this very easily. zMapper allows you to define new room shapes/icons. But there is no way to "merge" a new shape (like a red outline) with an existing shape. All you could do is create a new room type with a shape with a red outline, and then use the COM zMapper scripting to change the current room to the new type.

For example, if you create a new room kind in zMapper with the red outline shape, and this is kind #8 (for example), then you could script:

%map.CurRoom.KindID = 8

to change the kind index of the current room.

Take a look at the zMapper help file in the section "Using zMapper with zMUD" for reference details on the COM scripting for zMapper.